We present results of measurements of cosmogenic 10Be, 26Al and 36Cl, and the indigenous
(intrinsic) concentrations of the stable elements Be, Al and Cl in 120–200 kyr old corals from Barbados and
Puerto Rico. The concentration levels of these radionuclides in the corals lie in the range 104 to 108 atoms/g.
A comparison of the measured nuclide concentrations with those expected to be produced in the corals by
nuclear interactions of energetic cosmic radiation shows that (i) the radionuclides 26Al and 36Cl are derived
from in situ cosmic ray interactions in the corals after their formation, but that (ii) the radionuclide 10Be owes
its provenance in the coralline lattice primarily due to incorporation of dissolved beryllium in seawater in the
lattice structure of the corals.

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for
mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to
examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment
denudation rates using in-situ-produced cosmogenic 10Be in fluvial sediments. Denudation processes in an agricultural
watershed (Cayagua´s River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared
using 10Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds
bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the
present-day sediment output of the Cayagua´s. This confirms the recent and presumably anthropogenic origin of the modern
high denudation rate in the Cayagua´s Basin and suggests that pre-agricultural erosional conditions were comparable to
those of the present-day Icacos.

We present a simple method for estimation of long-term mean denudation rates using in situ-produced
cosmogenic 10Be in fluvial sediments. Procedures are discussed to account for the effects of soil bioturbation, mass
wasting and attenuation of cosmic rays by biomass and by local topography. Our analyses of 10Be in quartz from
bedrock outcrops, soils, mass-wasting sites and riverine sediment from the Icacos River basin in the Luquillo
Experimental Forest, Puerto Rico, are used to characterize denudation for major landform elements in that basin.
The 10Be concentration of a discharge-weighted average of size classes of river sediment corresponds to a long-term
average denudation of = 43 m Ma^-1, consistent with mass balance results.

Increased sediment flux to the coastal ocean due to coastal development is considered a major threat to the viability of coral reefs. A
change in the nature of sediment supply and storage has been identified in a variety of coastal settings, particularly in response to European
colonization, but sedimentation around reefs has received less attention. This research examines the sedimentary record adjacent to
a coastal village that has experienced considerable land-use change over the last few decades. Sediment cores were analyzed to characterize
composition and sediment accumulation rates. Sedimentation rates decreased seaward across the shelf from 0.85 cm y1 in a nearshore
bay to 0.19 cm y1 in a fore-reef setting. Data reflected a significant (up to 2) increase over the last 80 years in terrestrial
sediment accumulating in the back-reef setting, suggesting greater terrestrial sediment flux to the area. Reef health has declined, and
increased turbidity is believed to be an important impact, particularly when combined with additional stressors.

Granite weathering and clay mineral formation impart distinct and interpretable
stable Si isotope (d30Si) signatures to their solid and aqueous products. Within a saprolite,
clay minerals have d30Si values ;2.0‰ more negative than their parent mineral and the
d30Si signature of the bulk solid is determined by the ratio of primary to secondary minerals.
Mineral-specific weathering reactions predominate at different depths, driving
changes in differing d30Sipore water values. At the bedrock-saprolite interface, dissolution
of plagioclase and hornblende creates d30Sipore water signatures more positive than granite
by up to 1.2‰; these reactions are the main contributor of Si to stream water and determine
its d30Si value. Throughout the saprolite, biotite weathering releases Si to pore waters
but kaolinite overgrowth formation modulates its contribution to pore-water Si. The influence
of biotite on d30Sipore water is greatest near the bedrock where biotite-derived Si
mixes with bulk pore water prior to kaolinite formation. Higher in the saprolite, biotite
grains have become more isolated by kaolinite overgrowth, which consumes biotite-derived
Si that would otherwise influence d30Sipore water. Because of this isolation, which shifts the
dominant source of pore-water Si from biotite to quartz, d30Sipore water values are more
negative than granite by up to 1.3‰ near the top of the saprolite.

Evaristo J. Review of Use of Isotopes in Studying the Natural History of Puerto Rico. University of Pennsylvania. 2012.

Abstract:

This review summarizes the earth and environmental science research papers in Puerto Rico that used isotopic techniques between 1965 and 2011. The range of applications in isotope-related research in Puerto Rico has grown steadily, led by the ubiquitous utility of stable isotope ratios in biogeochemical (δ13C, δ15N) and ecological (δ13C, δ15N, δD) research. Moreover, research in climatology has grown in recent years, spanning from the evaluation of the fidelity of isotope records (δ18O, δ13C) as an environmental proxy to the elucidation of multidecadal variability for paleoclimate reconstructions (δ18O and Sr/Ca). On the other hand, in addition to using isotope ratios, hydrological studies in Puerto Rico have also used trace element data to answer flow source (δD, δ18O, 87Sr/86Sr) and solute source (Ge/Si) questions, as well as in examining groundwater/surface flow relationships (222Rn). Finally, various isotope data have been used in trying to understand geomorphological (10Be, δ30Si) and geophysical (Pb, Nd, and Sr) phenomena. It is hoped that this review will be able to contribute to stimulating future interests in isotope-related research as applicable in the LCZO or Puerto Rico, in particular, and/or in comparable humid tropical settings, in general.

Geomorphology, the study of landscape
form and change, is at a crossroad. Quantification
of patterns on Earth’s surface has
revealed surprising and robust order. Transport
equations rooted in physics that relate
material flux of sediment to environmental
drivers have been derived and are capable
of simulating realistic-looking
topography
[Dietrich et al., 2003]. Yet despite this rapid
progress and an explosion of interest in the
field, scientists are unable to predict sediment
transport rates in rivers to better than
an order of magnitude—they can only qualitatively
anticipate the response of landscapes
to land use or climatic changes. In addition,
some argue that the sedimentary archive of
landscape evolution is dominated by randomness
[Sadler and Strauss, 1990], calling into
question researchers’ ability to reconstruct
environmental change from the rock record.
A major obstacle to progress is the lack
of understanding of the nature and origins
of “noise” in sediment transport. “Noise”
here refers to the seemingly random or inexplicable
fluctuations in transport rate that
can occur without any external perturbation.
Transport noise can obscure the relations
between cause and effect in landscape
evolution. Two key questions help to focus
research efforts: (1) How do noisy dynamics
arise in a steady environment? (2) How can
robust, ubiquitous patterns persist in a noisy
environment? Addressing these questions
will require the adoption of theoretical and
experimental approaches that are outside
the realm of traditional geomorphology.